Recent years, progress of the superconductive technology is remarkable, and, as its applicable field is expanded, development of a refrigerator of small size and high performance becomes inevitable issue. For such a refrigerator, light weight/small size and high thermal efficiency are required.
For example, in a superconductive MRI device and a cryopump, a refrigerator operating based on a refrigeration cycle such as a Gifford MacMahon system (GM system) or a Stirling system is used. Further, a high performance refrigerator is indispensable for a magnetic levitation train too, still further, for some single crystal growth devices, a refrigerator of high performance is being used. In such a refrigerator, inside a heat regenerator filled with a heat regenerating material, an operating medium such as a compressed He gas and the like flows in one direction to supply its heat energy to the heat regenerating material, and there expanded operating medium flows in the reverse direction to receive a heat energy from the heat regenerating material. As an recuperating effect becomes good through such a process, the thermal efficiency of the operating medium cycle can be improved, thereby, a further lower temperature can be realized.
As a heat regenerating material to be used for the above described refrigerator, conventionally, there has been mainly used Cu or Pb. However, since these heat regenerating materials become remarkably small in their specific heat at very low temperature of 20 K or less, the above described recuperating effect does not work sufficiently, resulting in difficulty in realization of a very low temperature.
Then, recently, in order to realize a temperature more close to the absolute zero degree, application of magnetic heat regenerating materials such as an Er--Ni based intermetallic compounds such as Er.sub.3 Ni, ErNi, ErNi.sub.2 (ref. Japanese Patent Laid Open No. HEI-1-310269) and RRh based intermetallic compounds (R: Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb) such as ErRh (ref. Japanese Patent Laid Open No. Sho-51-52378), all of which display a large specific heat at very low temperature, are under investigation.
Now, in an operating state of a refrigerator such as described above, an operating medium such as a He gas and the like passes through space between the heat regenerating material filled in the heat regenerator in such a manner that changes frequently its flowing direction under high pressure and with high speed. Therefore, a various kinds of forces including mechanical vibration are added on the heat regenerating material. Further, when a magnetic levitation train or an artificial satellite is equipped with a refrigerator, there operates a large acceleration on the heat regenerating material.
Thus, though various forces act on the heat regenerating material, since the above described magnetic heat regenerating materials consisting of the intermetallic compounds such as Er.sub.3 Ni and ErRh are brittle materials in general, due to the cause such as the above described mechanical vibration or acceleration during operation, there was a problem that they were prone to be pulverized. The pulverized fine particles hinder the gas sealing to adversely affect on the performance of the heat regenerator, thus, resulting in deterioration of the capacity of the refrigerator.
An object of the present invention is to provide a heat regenerating material which can be used at a very low temperature and is excellent in their mechanical performance against the mechanical vibration or the acceleration, and a refrigerator which enabled to exhibit an excellent refrigeration performance over a long term by using such a heat regenerating material. Further, the other object is to provide an MRI device, a cryopump, a magnetic levitation train, and a magnetic field application type single crystal growth device all of which are made possible to exhibit excellent performance over a long term by using such a refrigerator.